We have heard back from Lithium, who was able to reproduce the issue over on their end. This phenomenon appears to occur whenever a poster's internet connection times out when the message (and attachment) is being uploaded. Because of the nature of this trigger, however, they are unable to provide a fix for this problem. Instead, they did provide some workarounds that should prevent duplicates from being created in the future:
Please keep in mind that the size limit for file uploads is 15MB. As your file size increases, you are more likely to run into the issue of a network timeout and, therefore, a duplicate post being created.
If your connection DOES time out, please do NOT use the "Try Again" button or try to reload your browser. Instead, navigate back and repost.
Please make note of these items and if you do experience a timeout when posting, double check to make sure that a duplicate has not been created. If one has been posted, please notify an administrator of the matter so we can remove the extra message.
Thank you very much for all of your feedback and patience with this issue!